I don't know about C++, but 'double precision' in modern programming languages (e.g. Visual Basic) implies 64-bit, which is known in assembly as a 'quad word' (4*16-bit). The older Fortran codes, such as Plot3D, only use 32-bit precision ('double word') for floating point numbers. So 'double double precision' in Fortran may be equivalent to 'double precision' in C++.